and im now using a local network sim card i wanted to know why i CANT use my BB messanger ?
and how i will be able to use it ? what do i need to do ?
because in usa i CAN i have the data plan for unlimited .
thanks so much for helping.

Are you certain that you have the BlackBerry data plan on your roaming (local) SIM?

Not just any old data plan, you need the BLACKBERRY one

You could leave your T-Mobile SIM card in your 8900 and call them to activate international data. But like Jadey said, you must have a BlackBerry data plan on the SIM card you're using to get BB Messenger, email, etc.

please activate your international data roaming, but as far as i know, you must pay per kb.

I was under the impression that Tmobile only has an International email plan (as opposed to international data).

All good points. check because the bills could be high.